1. Field of the Invention
The present invention generally relates to thermal-insulating ceramic coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a deposition process that can be performed at relatively low surface temperatures to form a thermal barrier coating that exhibits improved resistance to spallation.
2. Description of the Related Art
Components within the hot gas path of a gas turbine engine are often protected by a thermal barrier coating (TBC) system. TBC systems include a thermal-insulating ceramic topcoat, typically referred to as the TBC, which is typically bonded to the component with an environmentally-protective bond coat. Bond coat materials widely used in TBC systems include overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth or reactive element such as hafnium, zirconium, etc.), and diffusion coatings such as diffusion aluminides, notable examples of which are NiAl and NiAl(Pt). Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques.
To be effective, TBC""s must have low thermal conductivity, strongly adhere to the surface it protects, and remain adherent through many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. For this reason, various coating systems have been proposed in which the TBC has enhanced strain tolerance as a result of the presence of porosity, microcracks and/or segmentation of the ceramic layer. Segmentation indicates that the ceramic layer has columnar grain boundaries or cracks oriented perpendicular to the surface of the component, such as that achieved with PVD processes such as electron beam physical vapor deposition (EBPVD). As is known in the art, a TBC having a columnar grain structure is able to expand with its underlying substrate without causing damaging stresses that lead to spallation. As a result, TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by EBPVD. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
EBPVD coaters, which typically require substrate temperatures in the range of throughput, and are therefore used in the production of gas turbine engine components. However, EBPVD coaters are relatively expensive and typically have long lead times for delivery as compared to other deposition equipment. Therefore, EBPVD coaters are not well suited for use in coating operations in which a relatively small number of components are to be processed, such as in the repair of gas turbine engine airfoils. While other PVD processes such as sputtering can be performed with equipment that costs much less than EBPVD coaters, and have capacities better matched to repair operations, sputtering technologies generally require hundreds of hours of deposition time to apply YSZ in thicknesses (e.g., about 125 micrometers) typically required for airfoils.
The present invention is a process for forming a ceramic coating on a component, such as a gas turbine engine component that will be subjected to a hostile environment. The coating has a columnar grain structure and is deposited by a reactive sputtering technique that significantly improves throughput, produces a coating with improved spallation resistance, and can be performed with an apparatus that can be significantly lower in cost than EBPVD equipment.
The process of this invention generally entails placing the component intended for coating in a chamber containing oxygen and an inert gas, heating a surface of the component to a temperature of about 50xc2x0 C. to about 400xc2x0 C., and then generating a metal vapor from at least one metal target using a microwave-stimulated, oxygen-containing sputtering technique. The metal vapor is then caused to condense on the component surface to form a metal layer, after which the metal layer is treated with a microwave-stimulated plasma to at least partially oxidize the metal layer and form an oxide layer having a columnar microstructure. The generating, condensing and treating steps can be repeated any number of times to form multiple oxide layers that together constitute the ceramic coating.
According to one aspect of the invention, the ceramic coating is almost fully substoichiometric zirconia as a result of using one or more zirconium-containing metal targets and the diffusion-limited oxidation reaction caused by the microwave-stimulated plasma. According to another aspect of the invention, if zirconium targets are used, a spall-resistant coating can be achieved with a coating consisting essentially of nonstabilized substoichiometric zirconia. Alternatively, one or more metal targets may consist of zirconium, yttrium, and incidental impurities, such that the ceramic coating consists essentially of zirconia at least partially stabilized by yttria, either or both of which are substoichiometric. According to yet another aspect of the invention, an yttrium metal layer can be deposited and at least partially oxidized in the same manner as and prior to the oxide layer, yielding an yttria layer on which the oxide layer is subsequently formed. According to one aspect of the invention, the ceramic coating is almost fully substoichiometric zirconia as a result of using one or more zirconium-containing metal targets and the diffusion-limited oxidation reaction caused by the microwave-stimulated plasma. According to another aspect of the invention, if zirconium targets are used, a spall-resistant coating can be achieved with a coating consisting essentially of nonstabilized substoichiometric zirconia. Alternatively, one or more metal targets may consist of zirconium, yttrium, and incidental impurities, such that the ceramic coating consists essentially of zirconia at least partially stabilized by yttria, either or both of which are substoichiometric. According to yet another aspect of the invention, an yttrium metal layer can be deposited and at least partially oxidized in the same manner as and prior to the oxide layer, yielding an yttria layer on which the oxide layer is subsequently formed.
Metal oxide coatings deposited in accordance with this invention have been determined to be significantly more spall resistant than coatings of the same materials deposited by EBPVD under conventional conditions. In addition, the coating deposition rate is significantly higher than conventional sputtering techniques. For example, an yttria-stabilized zirconia (YSZ) coating can be deposited with this process to a thickness of about 125 micrometers in approximately twenty hours by using multiple targets, and approximately eighty hours using a single target. In comparison, several hundred hours are required to deposit equivalent coatings using conventional sputtering techniques. Accordingly, the process of this present invention not only improves the spallation resistance of the resulting coating, but also significantly improves manufacturing economies, particularly for relatively low-volume coating repair operations. Another advantage of the sputtering process of this invention is that sputtering equipment can be purchased at lower cost and delivered and installed in a time frame shorter than that for EBPVD equipment.
Other objects and advantages of this invention will be better appreciated from the following detailed description.